Means for rapid charging and dynamic discharging of a capacitively charged electrical device

Abstract

A dynamic discharge circuit for a capacitively-charged electrical device includes, in one embodiment, a bipolar transistor placed in the discharge path, with the base of the transistor being connected so that conduction is held off by the voltage generated by an illuminated PV-diode. When the PV-diode is shut off, base-drive is immediately supplied through a base resistor in dynamic fashion by the very capacitive charge that is to be discharged. In another embodiment, the resistor is replaced with a diode-transistor combination, thus eliminating the delaying RC product inherent with the use of a resistor. In yet a further embodiment, an SCR placed in the discharge path is also dynamically driven. Discharge times on the order of about 5 microseconds are attained with the latter two embodiments and it is found that this discharge time is relatively constant regardless of the value of the capacitance to be discharged. Substantially improved charging time of the electrical device is provided in a circuit which includes a phototransistor which supplies charging current derived from the main power supply. The embodiments are described as being applied to the discharge of a capacitively-charged MOSFET in an optically-coupled solid state relay.

We claim: 
     
       1. A solid state drive circuit, comprising: (a) an LED;   (b) a PV-diode array optically-coupled to said LED;   (c) a first conductor connected to the anode end of said array for connection to the gate of a MOSFET;   (d) a second conductor connected to the cathode end of said array for connection to the source of said MOSFET;   (e) discharge means connected between said first and second conductors; and   (f) a phototransistor optically-coupled to said LED and connected between said anode end of said array and said gate of said MOSFET, said phototransistor having its base connected to said anode end, its emitter connected to said gate, and its collector connected to the drain of said MOSFET.   
     
     
       2. A solid state drive circuit, as defined in claim 1, further comprising a capacitor connected between said collector of said phototransistor and said source of said MOSFET. 
     
     
       3. A solid state drive circuit, as defined in claim 1, further comprising a resistor connected between said collector of said phototransistor and said drain of said MOSFET. 
     
     
       4. A solid state drive circuit, as defined in claim 1, further comprising a diode connected between said collector of said phototransistor and said drain of said MOSFET. 
     
     
       5. A solid state drive circuit, as defined in claim 1, wherein said discharge means comprises: (a) a first transistor having its collector connected to said anode of said PV-diode array and its emitter connected to said cathode of said PV-diode array;   (b) a second transistor having its base connected to the base of said phototransistor, its emitter connected to said emitter of said phototransistor, and its collector connected to the base of said first transistor; and   (c) a photovoltaic diode optically coupled to said LED and connected between said bases of said first and second transistors and said second conductor to hold off conduction of said first transistor when said photovoltaic diode is illuminated and to allow conduction of said first transistor when said second photovoltaic diode is not illuminated.   
     
     
       6. A solid state drive circuit, as defined in claim 5, further comprising a resistor connected between said collector and said base of said first transistor.